Acceleration time (ACC) in a Variable Frequency Drive (VFD) defines the programmed ramp used to increase the drive's output frequency toward its selected reference or ramp-scaling frequency. Deceleration time (DEC) defines the corresponding controlled reduction in frequency.

These parameters affect much more than how quickly a motor appears to speed up or stop. Motor current, accelerating torque, mechanical shock, product movement, regenerative energy and DC-bus voltage can all be influenced by the selected ACC and DEC ramps.

Variable Frequency Drives  ›  ACC / DEC Time  ›  S-Curve  ›  Automatic Ramping  ›  DC Bus  ›  Coast Stop  ›  STO

How does an acceleration ramp work?

If a drive is programmed for a 10-second acceleration ramp, it does not immediately apply the final frequency command to the motor. Instead, the output reference rises according to the programmed ramp profile, reducing abrupt mechanical loading on the motor and driven machine.

However, an ACC setting of 10 seconds does not guarantee that the real motor will always reach its target speed in exactly 10 seconds. Actual acceleration is affected by load inertia, available motor torque, current limit and the selected control method.

If the drive reaches its functional current limit during acceleration, many drives will reduce the acceleration rate rather than blindly follow the original ramp. The programmed ramp is therefore a commanded trajectory, not an unlimited torque command.

Why can a short acceleration time cause high current?

Accelerating a mechanical load more quickly requires greater accelerating torque. As inertia increases and the requested acceleration time becomes shorter, the additional torque demanded from the motor rises.

On a heavy conveyor, large flywheel, mixer or centrifuge, an aggressive ACC setting can therefore push the VFD into current limiting or, in more severe cases, an overcurrent fault.

A shorter ramp is not automatically a better ramp. Production-cycle requirements have to be balanced against motor torque, drive current and mechanical stress.

Is a very long acceleration ramp always better?

No. Excessively long acceleration can unnecessarily extend the machine cycle. On heavily loaded equipment it can also keep the motor in a high-slip, high-torque region for longer than necessary.

Pumps and fluid systems may benefit from a controlled ramp to reduce sudden hydraulic changes, while a loaded conveyor must still develop enough torque to move the material reliably.

Where does the energy go during deceleration?

A rotating motor and mechanical load contain kinetic energy. When the VFD forces the system to slow down quickly, the motor can operate as a generator and return energy toward the drive's DC bus.

If DEC time is too short, regenerative energy can charge the DC-link capacitors and raise DC-bus voltage. If the drive cannot control that energy, it can trip on DC-link overvoltage.

Some VFDs provide automatic deceleration-ramp adaptation and increase the real stopping time when the DC bus approaches its limit.

What if a longer DEC time is not acceptable?

Where the process requires a genuinely short stopping time, the regenerative energy has to be managed rather than ignored. A braking chopper and correctly sized braking resistor can dissipate the returned energy as heat.

Braking-resistor selection is not based on resistance alone. Peak braking power, braking energy, braking duration, repetition rate and duty cycle all affect the correct solution.

Ramp stop vs Coast Stop

With a Coast Stop, the drive removes active motor torque and allows the machine to slow according to inertia and friction.

With a ramp stop, the VFD actively reduces its output reference according to the programmed deceleration ramp. These stop methods therefore have different mechanical and DC-bus behaviour.

Is DC injection braking the same as a deceleration ramp?

No. DC injection braking applies DC current to the stator to create a stationary magnetic field and braking torque.

It can be useful near zero speed or in selected stopping applications, but excessive DC level or braking duration can heat the motor significantly. It should not be treated as a universal replacement for a correctly configured DEC ramp.

Linear ramp vs S-Curve

A conventional linear ramp changes speed at an approximately constant rate, but acceleration itself changes abruptly at the beginning and end of the ramp. This rate of change of acceleration is known as jerk.

S-Curve control rounds the transitions into and out of acceleration and deceleration. This can reduce mechanical shock and improve motion quality on conveyors, elevators, material-handling systems and other sensitive machinery.

Too much S-Curve is not automatically better; excessive smoothing can make the process unnecessarily slow.

Why do some VFDs provide several ramp sets?

Modern drives can provide multiple ACC / DEC sets. One ramp may be used during normal production while another is selected for a different process state, maintenance mode or frequency range.

For example, ABB ACS580 supports two ramp sets and can switch between them through a digital input or automatically above a defined frequency. Inovance MD310 also provides multiple acceleration and deceleration time groups.

Do pumps, fans and conveyors use the same ramp?

No universal ramp time exists. A pump may need a controlled speed change to limit hydraulic disturbance. A high-inertia fan can require a long real acceleration time simply because of its stored mechanical energy.

A conveyor may require a ramp that avoids belt shock and product movement while still reaching production speed quickly enough. Generic advice such as “always set 5 seconds” should therefore be avoided.

Does a VFD ramp eliminate starting current?

A VFD can manage motor current much more effectively than direct-on-line starting, but acceleration still requires torque. If the mechanical load demands substantial accelerating torque, motor current can rise well above its light-load value.

Ramp selection should therefore be checked against actual motor current, required torque and drive overload capability, not the time setting alone.

Normal ramp stop vs STO

A normal STOP command and DEC ramp are process-control functions. Safe Torque Off (STO) is a safety-related drive function that prevents the drive from producing motor torque when correctly integrated into a safety system.

STO is not a deceleration ramp. It does not by itself guarantee that the machine reaches zero speed within a defined time.

Editor's note: I do not treat ACC and DEC as simple “how many seconds?” parameters. On acceleration I watch real current and whether the drive is reaching its torque or current limit. On deceleration I watch where the machine's stored energy is going. If the drive is tripping on DC-bus overvoltage, making DEC even shorter is not tuning — it is asking the power stage to absorb energy that has nowhere to go. Ramp settings sit directly between the mechanics of the machine and the power electronics of the drive.

Related topics

What is a VFD? — main variable-frequency-drive guide
What is S-Curve Control? — jerk and smooth motion profiles
What is Automatic Ramping? — adapting acceleration and deceleration behaviour
What is a DC Bus? — intermediate circuit where regenerative energy appears
What is Coast Stop? — stopping without an active deceleration ramp
What is DC Braking? — DC injection and braking behaviour
What is Regenerative Energy? — energy returned by a decelerating load
What is STO? — safety-related torque removal, not normal ramp stopping

In summary: ACC and DEC are not merely motor timing settings. Acceleration is limited by available torque and current, while deceleration is strongly affected by regenerative-energy and DC-bus management. Correct ramp times must therefore be chosen from the real motor, load, inertia, braking system and process requirements.